The invention relates to sequential firing of static power switches in general, and more particularly to the generation of successive commands into ON and OFF stages of successive transistors, or GTO power switches.
Most driver circuits, typically for inverters, which use transistors, or GTO's, have a built-in delay which performs one or more of the following three functions:
Underlap, i.e. a time delay interposed between the OFF stage and the ON state of successive static switches;
Minimum ON and OFF time Control for snubber discharge;
Noise filtering.
In most inverter configurations, two inverter switches are connected in series across a DC voltage or current source. If both switches are inadvertently gated ON simultaneously, undesirable consequences may result. To prevent this, a slight delay between the turning OFF of one switch and the turning ON of the opposite switch is introduced. This is usually referred to as "underlap". It is often implemented by adding deliberate delays in the driver circuits so that the turn-ON delay will be longer than the turn-OFF delay. If the control commands both switches in a complementary manner, the underlap will occur naturally.
Minimum ON and OFF times are necessary with switch configurations which use snubbers because, for proper operation, the snubber must have been set into a certain state before the next transition can occur. Also, noise filtering is usually required, because the high voltage slew across the control signal isolation (optocoupler, transformer, etc.) tends to inject noise into the sensitive part of the driver circuit.
A delay circuit in the prior art usually consists of an RC charging and discharging network and a comparator. Such circuits, however, cause variations in the turn-ON and turn-OFF times, which variations are detrimental for critical applications, such as high frequency inverters and PWM inverters. The error is due to a variation in the state of the timing capacitor voltage at the beginning of a command for either change of state. Such variation is dependent upon the time that has elapsed since the previous change of state. In principle, no problem occurs at low frequencies. However, the delays may not be repeatable if the transistions are too closely spaced.